Cochlear implants are implantable systems which can provide hearing to profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids which mechanically apply an amplified sound signal to the middle ear, a cochlear implant provides direct electrical stimulation to multiple implant electrodes that excite the acoustic nerve in the inner ear. Most current cochlear implant electric stimulation coding strategies represent a sound signal by splitting it into distinct frequency bands and extracting the envelope (i.e., energy) of each of these bands. These envelope representations of the acoustic signal are used to define the stimulation amplitude of each electrode.
One current approach, the Fine Structure Processing (FSP) coding strategy, commercially available in the Med-E1 OPUS 1 and OPUS 2 speech processors, analyzes the phase of the band pass signals and synchronizes the stimulation pulses with specific events in the phase of the corresponding electrode. In FSP coding, time events are defined using the zero crossings of the band pass signal where all system channels are stimulated sequentially in a predetermined order (a “stimulation frame”). The stimulation rate or grid respectively of each channel is generally defined by the sum of the pulse durations and the pauses between consecutive stimulation pulses. The frame rate (i.e. the repetition rate) of one stimulation frame equals the stimulation rate or grid of each channel, typically 1000-2000 Hz.
FSP coding uses Channel Specific Sampling Sequences (CSSS), described, for example, in U.S. Pat. No. 6,594,525 (incorporated herein by reference) to represent the temporal information in the band pass signal. After a zero crossing in the band pass signal, a specific CSSS is started at the assigned electrode. The temporal accuracy is determined by the grid that is equal to the frame rate in FSP coding. This accuracy allows for coding temporal fine structure information up to several hundred Hertz. The temporal accuracy of CSSS in FSP is mainly defined by the pulse durations, i.e. at high pulse duration the accuracy of CSSS is low and the maximum frequency coded temporally is low as well.
Higher temporal accuracy of stimulation pulses can be achieved in an temporal fine structure coding strategy using CSSS together with the use of selected channel stimulation groups, as described, for example, in U.S. Pat. No. 7,283,876 (incorporated herein by reference). Different types of channels are defined (e.g. CSSS channels and envelope channels) and certain channels have to be grouped. For example, all the CSSS channels are placed into one or more groups in which some of the groups are repeated more often during a given stimulation frame. And within a given group, one or more of the channels can be stimulated simultaneously. This results in a temporal grid of CSSS stimulation which is a multiple of the frame rate. Improved temporal accuracy of the CSSS allows coding of phase information (up to about 1000 Hz) based on a high temporal grid using short pulse durations. With high pulse durations, temporal accuracy and frame rate (i.e. the rate of high frequency envelope channels) are reduced again. Most feasible combinations of CSSS, selected groups, and simultaneous stimulation, will have some mismatch between average CSSS rates of the highest CSSS channel and neighboring envelope channels. In such temporal fine structure coding strategies, a certain number of requested stimulation pulses are deselected. The number of deselected stimulation pulses (mainly within CSSS channels) is higher with higher pulse durations, which might lead to a loss of temporal information.
The current literature describes three other approaches that provide some temporal fine structure information. Peak Derived Timing (PDT) was described in Vandali et al., Pitch Ranking Ability Of Cochlear Implant Recipients: A Comparison Of Sound-Processing Strategies, J Acoust Soc Am. May 2005; 117(5):3126-38 (incorporated herein by reference). The PDT coding was experimentally used in cochlear implant users and derived the timing of stimulation pulses from the positive peaks in the band pass signals. Timing of the pulses was managed by an arbitration scheme which delayed or advanced simultaneously requested stimulation pulses. No refractory behavior was implemented in this algorithm.
Asynchronous Interleaved Sampling (AIS) was described in Sit et al., A Low-Power Asynchronous Interleaved Sampling Algorithm For Cochlear Implants That Encodes Envelope And Phase Information, IEEE Trans Biomed. Eng. January 2007; 54(1): 138-49 (incorporated herein by reference). The AIS strategy used asynchronous extraction of time events from band pass signals, but lacked any handling of interleaved stimulation pulses, which are a necessary part of a usable cochlear implant sound coding strategy.
Spike-based Temporal Auditory Representation (STAR) strategy is based upon an auditory model as described, for example, in Grayden et al., A Cochlear Implant Speech Processing Strategy Based On An Auditory Model, Proceedings of the 2004 Intelligent Sensors, Sensor Networks and Information Processing Conference, 14-17 Dec. 2004: 491-496 (incorporated herein by reference). The STAR approach, somewhat like CSSS, extracted the pulse timing from the zero crossings of the band pass signals. In this strategy ‘spike timing contentions’ are resolved by systematically shifting stimulation pulses to different time instances around the zero crossing. No details about the algorithm are given. The average stimulation rate on high frequency channels is restricted, but no details about the mechanism are given in the publication.